development or health? don't know which - help!

a strange one this. my dd (2) has developed over the last 6 months a hairy back. It's light brown down than goes over her shoulders and down her spine. At first we weren't that concerned as we thought it'll just fall out when it's ready to. Here's the weird bit. I was washing her in the bath and wanted to check her knees which are always scabby/ in the process of healing from last fall (!). I then noticed a tuft of dark coarse hair on her knee. It freaked me out a bit to be honest as you don't normally feel that texture of hair on little ones. It's not so obvious as it must have been there a while and I just noticed it tonight but it's truly strange. What's it doing there? Is it linked to her back hair? I also have a DS who from being super bald all over is now developing a hairy back too!!! Neither my DH or I are hairy so we're flummoxed and of course I'm a bit worried it's something hormonal? Thoughts anyone?